Re: [Ntp] NTPv5 and anycasting

[Watson Ladd <[e-mail]>]

As the operator of one of the biggest anycast NTP deployments in the
world, how about not?

At least in our case,  we use ECMP on the router, so if an association
uses a persistent port, it will get to the same server modulo
announcement changes due to server failure. I'd be happy to
collaborate in measurement research to quantify the magnitude of any
such effects when this is not the case. Route flaps are pretty rare,
so that's not much of an issue. I know the synchronization inside the
datacenter is quite good.

If we had to use DNS based load balancing that would be equally
nondeterministic, but would be significantly slower to adapt to server
faults, more susceptible to improper DNS or association caching by
clients, and complicated for boring architectural reasons. Anycast
scales much better without IP address consumption (for v4), while DNS
gives more flexibility in load balancing in the working case. DNS
would enable clients to use multiple different points of presence, and
figure out which has the best time, vs. being subject to the whims of
BGP. I don't know how much of an advantage that gives in realistic
scenarios.

In practice I think many single-homed servers are in fact anycasted
over a small number of servers behind a single Internet facing router
to provide redundancy. Once you're in the world of uptimes greater
than that of a single server, these sorts of setups are essential.

We also would not be able to join the pool without lots of operational issues.

I'm happy to collaborate in answering these questions: there are no
real secrets in the time service, and the NTP community has been very
helpful. We'd love to return the favor.

Sincerely,
Watson Ladd